This invention relates generally to ophthalmic spectacle lenses and more specifically to ophthalmic spectacle lenses which incorporate a concave aspherical, atoroidal surface to correct ocular astigmatism in both principal meridians for a minus range of lens powers.
The design of ophthalmic lenses is very important in the field of vision correction. Such lenses are used to assist an eye which is unable to properly focus. Ordinarily, the design of a lens is a tradeoff between a variety of factors, including weight, lens thickness and optical errors. Lenses which optimize these factors are known as best-form lenses
Jalie, U.S. Pat. No. 4,289,387, describes ophthalmic spectacle lenses of reduced thickness and weight with improved optical properties made possible by aspherizing the major surface of the lens. Prior to this patent, lenses incorporated only spherical or toroidal surfaces. It was discovered that by making the concave surface for a minus lens hyperboloidal, rather than the traditional spherical surface, the surface astigmatism inherent in the hyperboloidal surface could neutralize the aberrational astigmatism of oblique incidence. Corrections for varying degrees of oblique astigmatism could be corrected by varying the eccentricity of the hyperboloidal surface. Thus, by using a hyperboloidal surface on the lens, it was possible to obtain improved overall lens performance and characteristics.
Normally, an aspherical surface may be chosen to eliminate aberrational astigmatism in the case of a spherical lens. However, if a spherical lens is to incorporate a correction for ocular astigmatism, it should include a cylindrical or toroidal surface on the opposite side of the lens. But by incorporating such a surface in the lens, the eccentricity chosen for the spherical power can be correct for only one principal meridian of the lens, for example, the vertical or horizontal, but not both. Thus, there presently exists no practical method to overcome the problem of correction of ocular astigmatism for both principal meridians of minus-powered lenses.